This invention relates to induction motors, and in particular to permanent split capacitor induction motors rated at 460 volts AC. The notation "permanent split capacitor" is a term applying to a single phase induction motor having a main winding arranged for direct connection to a source of power and an auxiliary winding connected to a capacitor, the capacitor and auxiliary winding being electrically connected to a source of power during both starting and running operations. While the invention is described in detail with respect to 460 volt motors, those skilled in the art will recognize the applicability of the invention to a variety of high voltage motors, for example, rated between 440 to 600 volts.
Induction motors having voltage ratings over 440 volts have created operating problems not prevalent in motor designs operable at lower rated voltages. For example, it is known that high voltages between the windings of an induction motor can cause break down of the insulation system normally inserted between the windings, resulting in motor failure. The insulation system utilized with the majority of motors presently available in the art makes use of insulation ladders between coil sets of the motor windings. That is to say, insulation is placed between the coil sets of at least the auxiliary and the main windings whenever the coil sets share slots in the stator assembly. That insulation commonly has an axial length which extends inwardly from the ends of the stator and has a plurality of slits cut in it which permit insertion of the insulation in the slots. The slits give the insulation the appearance of a ladder. Consequently, the insulation is known in the art as an insulation ladder. Insulation ladders conventionally are constructed from material available from the E. I. duPont de Nemours and Company under the trademarks "Mylar" and "Nomex". In general, Nomex has a higher dielectric constant than Mylar, but is more susceptible to physical puncture during motor manufacture.
The use of insulation ladders in induction motors is an old expedient. In the past, it was considered necessary to use insulation between coil sets of all motor windings in order to prevent motor failure. Advancements in wire coatings used in conjunction with wire manufactured for dynamoelectric machines, known in the art and for the purposes of this specification as magnet wire, have increased the dielectric constant of the wire films to the point where the wire itself provides an effective insulative barrier between the turns of the coil sets forming the windings, particularly at the most common voltage sources applied to dynamoelectric machines, that is, 115 or 230 volts. It therefore has been the practice to eliminate the insulation ladders in induction motors operating at 115 and 230 volts, assuming other operating conditions do not permit voltages between coil sets to exceed the insulation breakdown voltage of the insulation film of the wire. However, wire film insulation alone is insufficient to protect against a short circuit between coil sets of the winding as the voltage between coil sets approaches 400 volts. Since 460 volt induction motors operate at voltages higher than the insulation breakdown voltages of the wire insulation, elimination of the insulation ladders for these motors was believed unfeasible.
The invention disclosed hereinafter makes the elimination of insulation between coil sets of motors having rated voltages above 440 possible by constructing the main winding in two parts, and electrically connecting the auxiliary winding across one main winding part. The auxiliary winding is placed in the stator assembly so that it is adjacent the main winding part across which it is connected. The auxiliary winding also is separated from the second main winding part by the first main winding part. This arrangement means that the voltages between coil sets of the winding all are within the voltage capabilities for the insulation films utilized in conjunction with the coil sets. Insulation between the coil sets thus may be eliminated, resulting in a considerable reduction in motor manufacturing costs and providing a more reliable motor design in that high voltages between coil sets are eliminated.
One of the objects of this invention is to provide a 460 volt rated induction motor which eliminates the use of insulation ladders between coil sets of the winding for the motor.
Another object of this invention is to provide a more economical motor design for an induction motor rated between 440 and 600 volts AC.
Yet another object of this invention is to provide a more reliable induction motor operable at 460 volts AC.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.